Lift truck construction

ABSTRACT

A lift truck includes a wheeled vehicle having divided longitudinal frames which may be extended or retracted for adjusting the longitudinal spacing between the front and rear wheels. A forklift carried by the vehicle is arranged to actuate a sensing device such as a dynamometer to measure the load to be lifted and the dynamometer is connected to a system to actuate means for shifting the divided frame elements carrying the wheels for the purpose of extending or retracting the wheel chassis in accordance with the value of the load to be lifted.

United States Patent Inventor Erich Hahn Reutlingen, Germany Appl. No. 810,549 Filed Mar. 26, 1969 Patented Feb. 23, 1971 Assignee Firma Ernst Wagner Apparatebav Reutlingen, Germany Priority Apr. 18, 1968 Austria 8A3788/68 LIFT TRUCK CONSTRUCTION 13 Claims, 4 Drawing Figs.

Int. Cl. Field of Search References Cited UNITED STATES PATENTS 2,320,601 6/1943 Howell 2,767,394 10/1956 Arnotetal 2,851,171 9/1958 Martin et a1. 2,916,172 12/1959 Lockg 214/674 FOREIGN PATENTS 1,148,019 7/1957 France 214/672 751,269 6/1956 Great Britain 214/672 Primary Examiner-Gerald M. Forlenza Assistant Examiner-Frank E. Werner Attorney-McGlew and Toren ABSTRACT: A lift truck includes a wheeled vehicle having divided longitudinal frames which may be extended or retracted for adjusting the longitudinal spacing between the front and rear wheels. A forklift carried by the vehicle is arranged to actuate a sensing device such as a dynamometer to measure the load to be lifted and the dynamometer is connected to a system to actuate means for shifting the divided frame elements carrying the wheels for the purpose of extending or retracting the wheel chassis in accordance with the value of the load to be lifted.

PATENTED FEB 23 1971 sum 2 [1F 2 INVENTOR ER #4 mv ATTORNEYS LIIFT 'rnucrr CONSTRUCTION SUMMARY OF THE INVENTION This invention relates, in general, to the construction of lifting and conveying trucks and, in particular, to a new and useful forklift truck having extensible side frame members for changing the spacing between the front and rear wheels and which includes dynamometer means for sensing the value of n the load to be lifted and for adjusting the wheel spacing accordingly.

It is important to have an adjustable wheelbase for lift trucks having a center of gravity which extends to the front or rear of the vehicle when the load is engaged. Principally, it is a prerequisite for lift trucks that are used in locations having a minimum amount of space and also where great load moments are to be encountered. The ability of such trucks to handle such great loads can be achieved by varying the wheelbase but trucks having a variable wheelbase have been used so far only to a very small extent. This is due to the fact that the driver cannot exactly estimate the load moment and then set the correct wheelbase. Consequently, there is a danger that the vehicle will tip when the wheelbase is not correctly set.

In accordance with the present invention, there is provided a truck construction which includes an adjustable wheelbase which can be used in practice without any difiiculties because it includes a dynamometer between the load carrier and the treads of the vehicle wheels which can be employed for determining the size of a load to be lifted. The driver can readily determine whether the load moment corresponds to the respective wheelbase setting by observing the dynamometer and he may increase the wheelbase .setting if necessary. Thus, the tipping dangers which were inherent in vehicles of this nature is eliminated.

Preferably the dynamometer is designed as a hydraulic cylinder having a movable piston andthe hydraulic cylinder is connected through a line to an indicator arranged in the drivers cabin. However, the dynamometer can also be arranged, for example, between the chassis and the wheel axles. In those lift trucks which include a supporting frame or mast which may be tilted by a tilting device, the dynamometer is arranged on the tilting device itself. For example, it can be connected between the tilting device and the mast at the center of gravity of the tilting device on the chassis. A special simplification is obtained if the hydraulic cylinder for tilting the mast is designed as a dynamometer. I

In accordance with another feature of the invention, the dynamometer is connected to an operating mechanism or switching device for changing the wheelbase in accordance with the load which is sensed and this eliminates any possible errors of judgement by the driver. The switching mechanism is advantageottsly arranged so that the dynamometer will prevent the lifting of a load at a certain load moment when the wheel base does not correspond to the load moment sensed or when the maximum admissible load moment is exceeded. In this way, it is possible to adapt the wheelbase automatically to the ioacl to be lifted. In order to provide greater variation possibilities, several switching devices, which act on mechanism to move the wheelbase elements sense the position of the relatively shiftable wheel frame portions and provide a corresponding extension or retraction of these portions in accordance with the load requirements.

In order to prevent a response of the switching mechanism if a wheelbase corresponding to the load already exists, locking devices are provided which act on. one or several of the switching devices in dependence on the orientation of the adjustable parts of the wheelbase. In a preferred embodiment, cam strips or similar devices designed as blocking elements are arranged on one of the mutually displa'ccable chassis parts and they cooperate with the movable switching devices arranged on the other part, for example, at a location perpendicularly to the longitudinal axis of the truck. Depending on the position in which the cam strips or other devices are located relative to the respective switching devices, there will either be a release or a blocking of the switching devices.

The switching devices are advantageously designed as electrohydraulic switches having a hydraulic part which is connected with the dynamometer and having an electrical part which is connected into the circuit of one or several drives for shifting the relatively movable parts of the chassis. In order to prevent the electrohydraulic switches from responding to shock pressures during the driving operation, damping elements, particularly chokes, are provided in the hydraulic feedlines. The electrohydraulic switches are preferably connected with several switches of which at least one is inserted into the circuit of the wheelbase drive for shifting the relatively movable parts of the wheelbase in order to achieve the automatic adjustment of the wheelbase to the desired wheel spacmg.

Accordingly, it is an object of the invention to provide a forklift truck having an adjustable wheelbase and which includes dynamometer means for sensing a loadtp be lifted for the purpose of determining the spacing between the front and rear wheels for adequately supporting and lifting a given load.

A further object of the invention is to provide a forklift truck which includes a forklift portion which advantageously actuates dynamometer means, in accordance with the size of the load being lifted, to determine the load size; the dynamometer means being, for example, a hydraulic drive for tilting the support mast or frame of the truck load carrier.

A further object of the invention is to provide a forklift truck which is simple in design, rugged in construction, and economical to manufacture.

The various features'of novelty which characterize the invention are pointedput with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which there are illustrated and described preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, in particular, the invention embodied therein comprises a forklift truck having a chassis frame 1 over which is mounted a driver's seat and motor enclosure 2. The chassis frame 1 includes side frame members 4 and S, which may be adjusted relative to each other in a longitudinal direction and which are arranged on each side of the chassis frame. The chassis frame 1 may be elongated or contracted in the directions of the double arrow 3 in order to vary the spacing between the front wheel set 8 and the rear wheel set 9. The side frame member 4 advantageously carries the main drive motor below the drivers seat and it is shifted on the frame 4 either toward or away from the lifting element or carrier 10 at the front of the vehicle for the purposes of providing a counter load of a size sufficient to overcome the weight of the load being lifted. The shifting of the side frame members 4 and 5 relative to each other is accomplished by means of a hydraulic piston and cylinder combination com prising a cylinder 6 having a piston 7 slidable therein which are connected between the two parts 4 and 5 to provide for the shifting thereof and the adjustment of the spacing between the forward wheels 8 and the rear wheels 9.

In the embodiment illustrated in FIG. 1, the forklift truck includes a mast or lifting frame 11 having the carrier 10 associated therewith which advantageously may be raised and lowered thereon. The mast 11 can be tilted by means of a hydraulic cylinder 12 having a piston 13 slidable therein which is connected between the chassis and the lower end of the mast 11 to permit it to pivot about its pivotal mounting 16. In a preferred arrangement, the cylinder '12 is designed as a dynamometer and when the carrier engages the load and the mast 11 is tilted to engage the carrier to lift the load, the amount of the load can be readily determined by the lifting force which is required. The dynamometer cylinder is provided with the usual hydraulic feed and discharge lines (not shown) and it is connected by a line 35 as indicated in FIG. 2 to a plurality of electrohydraulic switches generally designated 14 which are arranged at spaced locations along the length of one of the frame members, for example, the frame member 4.

A load 15 bearing on the load carrier 10, as represented by broken lines if FIG. 1, will produce a moment of rotation about the mast 11 and its supporting pivotal mounting 16 which will be manifest in a corresponding pressure in the hydraulic cylinder 12. The pressure which is sensed can be used to determine the value of the load 15 and can also be used to actuate the electrohydraulic switches 14 or an indicator (not shown) which is located adjacent the drivers seat 2.

The position shown in solid lines in FIG. 1 is retracted state of the chassis and the dotted line position shows the maximum extended state. The extended state is assumed when a particularly heavy load is to be raised.

In the embodiment indicated in FIG..2, the switches 14 include electrical portions or contact elements 18 which are connected to an electrical circuit 19 which is provided for the control of the operation of the lifting and driving cylinder 12. Each switch 14 also includes corresponding piston and cylinder arrangements 21, 22 and 23 which are set for response at different load movements. For example, the switch cylinder 21 closer to the rear axle responds to a lower load moment and the following switches 22 and 23 are set to the next higher values progressively. As soon as the switch 21 responds, its electrical part opens to affect the control circuit 19 which is connected to the lifting cylinder 12 and the driving cylinder 6. If the wheelbase setting corresponds to the load which is to be lifted, then the switch 21 is prevented from responding by a strip or cam 24 which is carried on the frame part5 and moves with the frame part 5 relatively to the frame part 4 of the chassis. The movement of the electrohydraulic switch on which the strip 24 bears is blocked so that the electrical part 18 cannot close the circuit 19 to effect a further drive of the cylinder 6 to either extent or retract the frame parts 4 and 5.

In the embodiment illustrated in FIG. 3, hydraulic switches 14' are provided which correspond to the switches 14 but which include a reversing switch portion engageable in a cir cuit 26 in series with the circuit 19' which is adapted to be connected to actuate mechanism for the lifting cylinder 12 and the driving cylinder 6. If the load is too low for one of the electrohydraulic switches 14 to respond, the latter will remain in the represented bottom position so that the lifting and driving drive to the respective cylinders 12 and 16 is connected and the wheel drive receives its voltage. As soon as one of the switches is actuated, the circuit 19' of the lifting and driving drive is stopped and the wheelbase drive is connected. The chassis part 5 thus is displaced relative to the part 4 and the strip 24 is correspondingly displaced in respect to the switches 14 back to an original position so that the increase of the wheelbase is completed and the lifting and driving drive is connected again. In this construction. an additional electrohydraulic switch 27 is provided at a location at which it will not cooperate with the locking strip 24 and which releases when a maximum load moment is attained which prevents the lifting and driving of the truck. A choke l7.is connected in the line 35 to the switches 14' to prevent actuation by impacts or collision shocks, for example.

In the embodiment indicated in FIG. 4, there is indicated a load dependent blocking device for the circuit closer of the wheelbase drive (which is not represented). A gearshift lever shaft 28 of the drive carries a shift lever 29 and it is provided with a locking cam 30 which is adapted to cooperate with a plunger 31 of a hydraulic switching device 32. The hydraulic switching device 32 is connected to the pressure line 35 which is connected to the cylinder 12, and it responds only at a pressure corresponding to a certain load moment. When the plunger 31 is in the represented position, that is, when the hydraulic switching device is released, the gearshift lever 29 can only be moved in one direction, that is, in the direction of the arrow 33. The opposite direction 34 is blocked. In this way, it is only possible to increase the wheelbase but not to decrease the length of the wheelbase. The embodiment of FIG. 4 operates similar to the embodiments of FIGS. 2 and 3 and several switching devices 32 are arranged along the length of one of the movable frame parts. Each switching device includes a locking cam 30 of a different length.

lclaim:

I. A lifting device comprising a wheeled chassis frame having an extensible and retractable side frame for changing the wheelbase length, a carrier supported on said chassis and being movable to lift a load, shifting means connected to said side frame for extending and retracting said frame to vary the wheelbase length, and dynamometer means connected to said carrier for actuation thereby in accordance with the load moment acting on said carrier and connected to said shifting means to extend and to retract said frame in accordance with the load movement acting on said carrier.

2. A lifting device, according to claim 1, including a carrier mast pivotally mounted on said chassis for pivotal movement about a substantially horizontal axis and extending upwardly from said chassis, said lift carrier being supported on said mast, hydraulic piston and cylinder means carried on said chassis and connected to said mast for pivoting said mast to effect lifting of said carrier with a load thereon, said dynamometer means comprising said hydraulic piston and cylinder means.

3. A lifting device, according to claim 2, wherein said dynamometer means includes switching means connected to said fluid piston and cylinder and being responsive to the pressure in said fluid cylinder to actuate means for moving said extensible and retractable side frame for changing the wheelbase length.

4. A lifting device, according to claim I, wherein said wheel chassis includes a front wheel set having a wheel axis and a rear wheel having a rear wheel axis, said dynamometer means being arranged between said chassis and said front and rear wheel axes.

5. A lifting device, according to claim 1, wherein said dynamometer means comprises a fluid pressure cylinder, and a piston slidable in said cylinder connected to said carrier.

6. A lifting device, according to claim 1, including means associated with said dynamometer means for indicating the load which is engaged by said carrier.

7. A lifting device, comprising a wheeled chassis having front and rear wheels and two chassis portions carrying respective ones of said front and rear wheels which are extensibly and retractably movable in relation to one another for changing the wheelbase length, a mast pivotally mounted on and extending upwardly from said chassis, said mast being pivotal about a substantially horizontal axis, a lift carrier supported on said mast and extending outwardly therefrom, and being movable to lift a load, a first fluid cylinder and piston combination connected between said chassis and said mast for pivoting said mast and shifting said carrier therewith for engaging and lifting a load and for lowering and disengaging a load, a second fluid piston and cylinder combination carried on said chassis and connected to said relatively movable chassis portions for shifting them relatively to change the wheelbase length, and dynamometer means connected to said carrier for measuring the load moment acting on said carrier and being connected to actuate said second fluid piston and cylinder combination for extending and retracting said two chassis portions in accordance with the load moment on said carrier. v

8. A lifting device, according to claim 7, wherein said dynamometer means comprises said first fluid cylinder and piston combination, a driving circuit for controlling the movement of said movable frame by actuating said second fluid cylinder and piston combination, and a plurality of switches connected in said operating circuit and being responsive to the pressure in said first fluid cylinderjand-piston combination as determined by the load engaged by said carrier to actuate said drive circuit by separate and distinct proportional amounts.

9. A lifting device comprising a wheeled chassis having front and rear wheels and two chassis portions carrying respective ones of said front and rear wheels which are extensibly and retractably movable in relation to one another for changing the wheelbase length, a mast pivotally mounted on and extending upwardly from said chassis, saidmast being pivotal about a substantially horizontal axis, a lift carrier supported on said mast and extending outwardly therefrom, and being movable to lift a load, a first fluid cylinder and piston combination connected between said chassis and said mast for pivoting said mast and shifting said carrier therewith for engaging and lifting a load and for lowering and disengaging a load, a second fluid piston and cylinder combination carried on said chassis and connected to said relatively movable chassis portions for shifting them relatively to change the wheelbase length, dynamometer means connected to said carrier for measuring the load moment acting on said carrier, said dynamometer means comprising said first fluid cylinder and piston combination, a driving circuit for controlling the movement of said movable frame by actuating said second fluid cylinder and piston combination and a plurality of switches connectedin said operating circuit and being responsive to the pressure in said first fluid cylinder andpiston combination as determined by the load engaged by said carrier to actuate said drive circuit by separate and distinct proportional amounts, at least one of said movable chassis portions including a blocking member preventing movement of said switches and hence actuation of said operating circuit for said second fluid cylinder and piston combination and to prevent relative movement of said movable chassis portions when said chassis portions are located at a proper orientation to produce the desired wheelbase in accordance with the load which'is' engaged by said carrier.

10. A lifting device, comprising a wheeled chassis having front and rear wheels and two chassis portions carrying respective ones of said front and rear wheels which are extensibly and retractably movable in relationto one another for changing the wheelbase length, a mast pivotally mounted on and extending upwardly from said chassis, said mast being pivotal about a substantially horizontal axis, a lift carrier supported on said mast and extending outwardly therefrom, and being movable to lift a load, a first fluid cylinder and piston combination connected between said chassis and said mast for pivoting said mast and shifting said carrier therewith for engaging and lifting a load and for lowering and disengaging a load, a second fluid piston and cylinder combination on said chassis and connected to said relatively movable chassis portions for shifting them relatively to change the wheelbase length, dynamometer means connected to said carrier for measuring the load moment acting on said carrier, said dynamometer means comprising said first fluid cylinder and piston combination, a driving circuit for controlling the movement of said movable frame by actuating said second fluid cylinder and piston combination, and a plurality of switches connected in said operating circuit and be responsive to the pressure in said first fluid cylinder and piston combination as determined by the load engaged by said carrier to actuate said drive circuit by separate and distinct proportional amounts, said plurality of switches comprising electrohydraulic switches each of said switches including hydraulic portions connected to said first fluid cylinder and piston combination and an electrical portion connected-to said operating device for said second flujdcylinder and piston combination.

11. A lifting device, according to claim 10, wherein said electrical switch portions of said switches includes a reversing circuit for effecting a reversing movement of said movable frame relative to said other frame.

12. A lifting device, according to claim 7, wherein said dynamometer means includes a plurality of electrohydraulic switches adapted to be connected to said first fluid cylinder and piston combination for operating said first fluid cylinder and piston combination and damping means associated with said switches.

13. A lifting device, comprising a wheeled chassis having front and rear wheels and two chassis portions carrying respective ones of said front and rear wheels which are extensibly and retractably movable in relation to one another for changing the wheelbase length, a mast pivotally mounted on and extending upwardly from said chassis, said mast being pivotal about a substantially horizontal axis, a lift carrier supported on said mast and extending outwardly therefrom, and being movable to lift a load, a first fluid cylinder and piston combination connected between said chassis and said mast for pivoting said mast and shifting said carrier therewith for engaging and lifting a load and for lowering and disengaging a load, a second fluid piston and cylinder combination carried on said chassis and connected to said relatively movable chassis portions for shifting them relatively to change the wheelbase length, dynamometer means connected to said carrier for measuring the load moment acting on said carrier, said dynamometer means including a hydraulic switch connected to said first fluid cylinder and piston combination and being movable in accordance with the pressure in said first fluid cylinder and piston combination as determined by the load which is being lifted by said carrier, said hydraulic switch including a piston arm portion which projects outwardly in accordance with the pressure of said first cylinder and piston combination, a hydraulic control for said second fluid cylinder and piston combination including a pivotal shift lever having a mounting shaft which is rotated by movement thereof and including a blocking cam thereon, said piston arm being movable to a position in which it blocks movement of said lever shaft in one direction but permits movement in an opposite direction for controlling the fluid operation of said second cylinder and piston combination. 

1. A lifting device comprising a wheeled chassis frame having an extensible and retractable side frame for changing the wheelbase length, a carrier supported on said chassis and being movable to lift a load, shifting means connected to said side frame for extending and retracting said frame to vary the wheelbase length, and dynamometer means connected to said carrier for actuation thereby in accordance with the load moment acting on said carrier and connected to said shifting means to extend and to retract said frame in accordance with the load movement acting on said carrier.
 2. A lifting device, according to claim 1, including a carrier mast pivotally mounted on said chassis for pivotal movement about a substantially horizontal axis and extending upwardly from said chassis, said lift carrier being supported on said mast, hydraulic piston and cylinder means carried on said chassis and connected to said mast for pivoting said mast to effect lifting of said carrier with a load thereon, said dynamometer means comprising said hydraulic piston and cylinder means.
 3. A lifting device, according to claim 2, wherein said dynamometer means includes switching means connected to said fluid piston and cylinder and being responsive to the pressure in said fluid cylinder to actuaTe means for moving said extensible and retractable side frame for changing the wheelbase length.
 4. A lifting device, according to claim 1, wherein said wheel chassis includes a front wheel set having a wheel axis and a rear wheel having a rear wheel axis, said dynamometer means being arranged between said chassis and said front and rear wheel axes.
 5. A lifting device, according to claim 1, wherein said dynamometer means comprises a fluid pressure cylinder, and a piston slidable in said cylinder connected to said carrier.
 6. A lifting device, according to claim 1, including means associated with said dynamometer means for indicating the load which is engaged by said carrier.
 7. A lifting device, comprising a wheeled chassis having front and rear wheels and two chassis portions carrying respective ones of said front and rear wheels which are extensibly and retractably movable in relation to one another for changing the wheelbase length, a mast pivotally mounted on and extending upwardly from said chassis, said mast being pivotal about a substantially horizontal axis, a lift carrier supported on said mast and extending outwardly therefrom, and being movable to lift a load, a first fluid cylinder and piston combination connected between said chassis and said mast for pivoting said mast and shifting said carrier therewith for engaging and lifting a load and for lowering and disengaging a load, a second fluid piston and cylinder combination carried on said chassis and connected to said relatively movable chassis portions for shifting them relatively to change the wheelbase length, and dynamometer means connected to said carrier for measuring the load moment acting on said carrier and being connected to actuate said second fluid piston and cylinder combination for extending and retracting said two chassis portions in accordance with the load moment on said carrier.
 8. A lifting device, according to claim 7, wherein said dynamometer means comprises said first fluid cylinder and piston combination, a driving circuit for controlling the movement of said movable frame by actuating said second fluid cylinder and piston combination, and a plurality of switches connected in said operating circuit and being responsive to the pressure in said first fluid cylinder and piston combination as determined by the load engaged by said carrier to actuate said drive circuit by separate and distinct proportional amounts.
 9. A lifting device comprising a wheeled chassis having front and rear wheels and two chassis portions carrying respective ones of said front and rear wheels which are extensibly and retractably movable in relation to one another for changing the wheelbase length, a mast pivotally mounted on and extending upwardly from said chassis, said mast being pivotal about a substantially horizontal axis, a lift carrier supported on said mast and extending outwardly therefrom, and being movable to lift a load, a first fluid cylinder and piston combination connected between said chassis and said mast for pivoting said mast and shifting said carrier therewith for engaging and lifting a load and for lowering and disengaging a load, a second fluid piston and cylinder combination carried on said chassis and connected to said relatively movable chassis portions for shifting them relatively to change the wheelbase length, dynamometer means connected to said carrier for measuring the load moment acting on said carrier, said dynamometer means comprising said first fluid cylinder and piston combination, a driving circuit for controlling the movement of said movable frame by actuating said second fluid cylinder and piston combination and a plurality of switches connected in said operating circuit and being responsive to the pressure in said first fluid cylinder and piston combination as determined by the load engaged by said carrier to actuate said drive circuit by separate and distinct proportional amounts, at least one of said movable chassis portions including a blocking membEr preventing movement of said switches and hence actuation of said operating circuit for said second fluid cylinder and piston combination and to prevent relative movement of said movable chassis portions when said chassis portions are located at a proper orientation to produce the desired wheelbase in accordance with the load which is engaged by said carrier.
 10. A lifting device, comprising a wheeled chassis having front and rear wheels and two chassis portions carrying respective ones of said front and rear wheels which are extensibly and retractably movable in relation to one another for changing the wheelbase length, a mast pivotally mounted on and extending upwardly from said chassis, said mast being pivotal about a substantially horizontal axis, a lift carrier supported on said mast and extending outwardly therefrom, and being movable to lift a load, a first fluid cylinder and piston combination connected between said chassis and said mast for pivoting said mast and shifting said carrier therewith for engaging and lifting a load and for lowering and disengaging a load, a second fluid piston and cylinder combination on said chassis and connected to said relatively movable chassis portions for shifting them relatively to change the wheelbase length, dynamometer means connected to said carrier for measuring the load moment acting on said carrier, said dynamometer means comprising said first fluid cylinder and piston combination, a driving circuit for controlling the movement of said movable frame by actuating said second fluid cylinder and piston combination, and a plurality of switches connected in said operating circuit and be responsive to the pressure in said first fluid cylinder and piston combination as determined by the load engaged by said carrier to actuate said drive circuit by separate and distinct proportional amounts, said plurality of switches comprising electrohydraulic switches each of said switches including hydraulic portions connected to said first fluid cylinder and piston combination and an electrical portion connected to said operating device for said second fluid cylinder and piston combination.
 11. A lifting device, according to claim 10, wherein said electrical switch portions of said switches includes a reversing circuit for effecting a reversing movement of said movable frame relative to said other frame.
 12. A lifting device, according to claim 7, wherein said dynamometer means includes a plurality of electrohydraulic switches adapted to be connected to said first fluid cylinder and piston combination for operating said first fluid cylinder and piston combination and damping means associated with said switches.
 13. A lifting device, comprising a wheeled chassis having front and rear wheels and two chassis portions carrying respective ones of said front and rear wheels which are extensibly and retractably movable in relation to one another for changing the wheelbase length, a mast pivotally mounted on and extending upwardly from said chassis, said mast being pivotal about a substantially horizontal axis, a lift carrier supported on said mast and extending outwardly therefrom, and being movable to lift a load, a first fluid cylinder and piston combination connected between said chassis and said mast for pivoting said mast and shifting said carrier therewith for engaging and lifting a load and for lowering and disengaging a load, a second fluid piston and cylinder combination carried on said chassis and connected to said relatively movable chassis portions for shifting them relatively to change the wheelbase length, dynamometer means connected to said carrier for measuring the load moment acting on said carrier, said dynamometer means including a hydraulic switch connected to said first fluid cylinder and piston combination and being movable in accordance with the pressure in said first fluid cylinder and piston combination as determined by the load which is being lifted by said carrier, said hydraulic switch including a pistoN arm portion which projects outwardly in accordance with the pressure of said first cylinder and piston combination, a hydraulic control for said second fluid cylinder and piston combination including a pivotal shift lever having a mounting shaft which is rotated by movement thereof and including a blocking cam thereon, said piston arm being movable to a position in which it blocks movement of said lever shaft in one direction but permits movement in an opposite direction for controlling the fluid operation of said second cylinder and piston combination. 